Digital cameras utilizing high-resolution electronic imaging sensors typically require high resolution optical components such as lenses. In addition, the lenses generally must be very compact, so that they can be incorporated into devices such as palm-sized computers, cellular telephones, and the like.
Lenses for digital cameras generally have several individual lens elements. The lens elements are typically spherical, and usually create spherical aberration. Coma, distortion, and field curvature are also common optical aberrations that occur in the imaging process of a typical lens. A large number of lens elements are generally required in order to balance the inherent optical aberrations. Lenses having a large number of lens elements tend to be large, heavy, and expensive to manufacture. The cost of these lenses increases with the number of lens elements, and also results in increased costs in assembling and mounting the lens elements in a lens cell.
Further, conventional lenses commonly use one or more aspheric lens elements, each of which has one or two non-spherical surfaces. The aspheric lens elements are made of plastic or glass. Aspheric glass lens elements may be produced through glass molding methods, but the cost is high compared to aspheric plastic lens elements. In addition, aspheric glass lens elements generally have poor resistance to strong impact, and are easily damaged. Furthermore, glass lens elements are heavier than plastics lens elements, and militate against the trend toward lightweight digital cameras. In comparison, aspheric plastic lens elements may be easily produced by plastic injection molding, and are relatively inexpensive. A plastic lens element is light in weight, and does not easily break.
A typical lens having only spherical lens elements is disclosed in U.S. Pat. No. 6,282,033. The lens includes four lens elements. A first lens element is biconvex, a second lens element is biconcave, a third lens element is a meniscus, and a fourth lens element is biconvex. All of the lens elements are made of glass, and all their surfaces are spherical. Such spherical glass lens elements are relatively easy to make. However, the overall weight of the lens is unduly great. In addition, the lens elements are easily damaged if they are subjected to shock, such as when a digital camera is dropped to the ground. Furthermore, the spherical lens elements are apt to produce spherical aberration, thereby diminishing the image quality.
A typical lens having both spherical lens elements and aspheric lens elements is disclosed in U.S. Pat. No. 6,441,971. The lens includes three lens elements. A first lens element is a glass spherical meniscus. A second lens element and a third lens element are aspheric and made of plastic. The combination of a glass spherical lens element and plastic aspheric lens elements can decrease the overall weight of the lens. However, the impact resistance of the lens is still rather weak, and the lens may be easily damaged if the corresponding digital camera is dropped to the ground.
Accordingly, what is needed is a lens for a digital camera which is light, inexpensive, and has strong impact resistance.